The invention relates to die casting of foam patterns for use in casting a two-cycle engine block in a lost foam casting process. More particularly, the invention relates to die casting of such patterns with a cylinder bore and a helical transfer passage which, because of its helical or inclined relation to the axis of the cylinder bore, includes an undercut portion which prevents one-piece die member removal in a direction along the axis of the cylinder bore. Also, the invention relates to such patterns which include a helical finger passage adjacent the transfer passage. Still more particularly, the invention relates to die members for die casting such patterns.
In the past, helical transfer passages were formed by a core which formed a major Portion of the transfer passage, including the undercut area, and which was pulled or removed entirely from the transfer passage along an axis at a slight angle relative to the cylinder axis before pulling or withdrawal of the supporting die member along the cylinder axis. Such removal involved a relatively long stroke length and increased the die casting cycle time. Because of the relatively complex shape of the core, it was difficult to fit the core accurately into the supporting die member so that the core could stroke freely while not allowing a large gap into which flash could enter. This construction also presented a maintenance problem.
Attention is directed to the following U.S. Patents:
______________________________________ Hale 4,092,958 Jun. 06, 1978 Pichl 4,129,103 Dec. 12, 1978 Neiman 4,243,093 Jan. 06, 1981 Osborn 4,632,169 Dec. 30, 1986 Martin 4,640,333 Feb. 03, 1987 Morris 4,657,063 Apr. 14, 1987 Trumbauer 4,691,754 Sep. 08, 1987 Hesterberg 4,721,149 Jan. 26, 1988 Trumbauer Re.31,488 Jan. 10, 1984 ______________________________________
Particular attention is directed to U.S. Pat. No. 4,632,169, to Osborn, which discloses a three-piece lost foam pattern. Osborn's transfer passages do not extend helically with respect to the cylinder axis.